Referring to the driving mechanism of a conventional bicycle, as shown in FIG. 9, a sprocket wheel 101 of relatively large radius (for example, about 10 cm-13 cm) which is directly coupled to a driving axle shaft 100 turned by driving the bicycle pedals, is arranged side by side with a single or double sprocket teeth sets, and a chain is connected over one of these teeth sets of the sprocket wheel 101 and over a sprocket wheel of the rear wheel so that the power is transmitted. The power increasing ratio or reducing ratio is determined merely in accordance with the difference of the number of sprocket teeth between the driving sprocket wheel 101 and the sprocket wheel of the rear wheel.
The prior art discloses various kinds of arrangements for multiple gearing ratios for the driving sprocket wheel and rear sprocket wheel, but these have some faults. In order to obtain a higher power ratio, the diameter of the driving sprocket wheel 101 has to be larger while the diameter of the rear sprocket wheel must be relatively smaller. However, in order to obtain a high torque and also lower power reduction ratios, the driving and rear sprocket wheels must have relative diameters contrary to that mentioned above.
Also, since the location of the driving axle shaft 100 of the bicycle is limited to a predetermined height (about 20 cm) from the ground level and the length of the driving arm 103 must be shorter than the axle height and typically does not exceed about 16 cm from the axis of the driving axle shaft 100, there has been a substantial limitation in the power increasing ratio as well as reducing ratio. If the diameter of the driving sprocket wheel 101 is increased, the weight of the driving sprocket wheel 101 becomes heavy, which increases the torque required to turn it and consumes unnecessary physical strength for running on a sloped road as well as on a level road.
The prior art also discloses various kinds of planetary gear arrangements for bicycles. However, the conventional devices have faults in that they are relatively bulky and of complicated construction which is costly and inconvenient for manufacture and assembly.
It is therefore a principal object of the present invention to provide a power driving device for a bicycle which provides a high power increasing ratio, yet is compact, inexpensive, and of a simple construction that would allow convenient manufacture and assembly. It is a specific object of the invention to employ a planetary gear type of power driving transmission for a bicycle.